This invention relates to a semiconductor doping process, and more particularly to a process for introducing dopant in a selected region in a semiconductor device wherein, without such dopant, the region would be prone to electrical breakdown. For the purpose of illustration, a preferred manner of practicing the invention is described in conjunction with the making of a field-effect, power-MOS type transistor wherein the invention has been found to offer particular utility.
In the packaging of a semiconductor device, such as a field-effect, power-MOS transistor, it is, of course, necessary to make electrical contact (through leads) to the several electrodes (gate, source, drain) in the device. Presently, the best-available wire-bonding technology to achieve this requires, because of the relatively gross size of even the smallest wire which is practically handleable, enlarged bonding pad areas in a device to provide an appropriate contact expanse for a wire lead.
In a power-MOS transistor, one of such enlarged bonding areas is provided for the gate, and inclusion of such an area, under certain circumstances, presents electrical breakdown problems. More specifically, for a given desired operational switching voltage in such a device, there is a maximum allowable spacing between what might be thought of as the electrical breakdown-prone zones in order to assure that breakdown does no occur. In a device of the type just mentioned, the zones referred to take the form of the so-called channels therein.
In a typical power-MOS transistor, a region which is especially problematic is that region which extends as an expanse beneath the gate contact area, which region is at least partially bounded by channels whose separation greatly exceeds the maximum permissible spacing to prevent breakdown. For example, in such a transistor, designed to operate with a switching voltage of 500-volts, the maximum allowable adjacent channel spacing is about 2-mils, yet in the gate contact area, it would be typical to find an adjacent channel spacing of about 12-mils. As was mentioned earlier, this greater spacing results as a consequence of having to provide a large enough contact area to enable a wire lead to fit.
A general object of the present invention, therefore, is to provide a unique doping process to achieve an improved semiconductor structure which, while allowing wide spacing between breakdown-prone zones to accommodate lead attachment, nevertheless substantially completely solves and avoids the problem of electrical breakdown.
In accordance with a preferred manner of practicing the invention, and as will be described more fully below, the invention proposes the creation of a specially doped region between such zones which, in effect, establishes an electrical connection with the zones in a manner which obviates electrical breakdown. In the case of a field-effect, power-MOS transistor, and referring for a moment specifically to what is known as an N-channel device where the channels contain a P- dopant, the invention proposes a technique for establishing a P- doped subregion extending between the adjacent channels underlying the gate contact areas, which subregion makes an electrical connection, and preferably a conductive connection, between the channels. This resulting structure eliminates the build-up and concentration of electrical fields which can cause debilitating electrical breakdown.
In general terms, the invention is practiced by forming over the region that extends between breakdown prone zones of the type mentioned, an oxide layer which contains a dopant that is electrically compatible with the N/P characteristic of the zones. Thereafter, dopant in this layer is driven therefrom into the region to create a doped subregion. Such driving action establishes an electrical connection between the newly doped subregion and the zones, which connection obviates electrical breakdown. Preferably, the steps of the invention are performed in such a manner that the penetration thickness of the newly doped subregion closely matches the penetration thickness of the zones between which it extends, with the boundaries of the subregion flowing with only minor curvature into the boundaries of the zones.
These and other objects and advantages which are attained by the invention will become more fully apparent as the description which now follows is read in conjunction with the accompanying drawings.